Life history stage, body size and morphology dominate fish critical swimming speed: a global synthesis

Critical swimming speed (Ucrit) is widely used to quantify prolonged swimming performance in fishes, yet available measurements remain concentrated in particular species, life history stages, morphotypes and experimental contexts, leaving broad-scale patterns of Ucrit variation poorly synthesized. Here, we compiled a global dataset of experimentally measured absolute and relative Ucrit (absolute Ucrit standardized by body length) from 1,345 experimental cases across 361 species and 387 publications. The results showed that body length was strongly associated with Ucrit, but this association varied between absolute and relative Ucrit and among life history stages. Absolute Ucrit increased with body length across all stages, more steeply in larvae than in adults and juveniles, whereas relative Ucrit declined with body length in adults and juveniles but increased weakly in larvae. Morphology provided further explanatory insight in adults and juveniles: elongated body forms and forked or lunate caudal fins were generally associated with higher Ucrit, while caudal peduncle throttling (CPt) was negatively associated with both absolute and relative Ucrit, and body lateral shape (BLs) was negatively associated mainly with relative Ucrit. Water temperature was positively associated with both Ucrit metrics after accounting for body length, life history stage, and ecosystem type, although temperature slopes varied among ecosystem groups. Overall, these results show that body-size scaling differs between absolute and relative Ucrit and that Ucrit variation is jointly associated with ontogeny, morphology, and temperature, providing a clearer basis for comparing swimming performance across fishes and interpreting Ucrit reference values in applied ecological and management contexts.